In some cases, a load (e.g. wheels of a vehicle) may require power at a rotary speed that is inefficient for the power source (e.g. an internal combustion engine). For example, some vehicle engines operate most efficiently in a narrow range of rotary speeds (e.g. RPM's).
A differential engine may be used to provide a variable speed conversion ratio between the output of a power source and the input to a load. This may permit the power source to operate at an efficient speed which is converted by the differential engine to provide the required input speed to the load.
According to some differential engine designs, a differential engine must be large to provide a large speed conversion ratio (whether positive or negative). For example, some differential engines include motor/generators for controlling the speed conversion ratio provided by the differential engine. In this case, a large speed conversion ratio may require more powerful (and typically larger) motor/generators.
It may be desirable for a system to provide a wide range of speed conversion ratios using a small differential engine.